The reactions and behavior of the eye have been under study for many years. In fact, as early as 1765 it was known that dilations and constrictions of the pupil were not all attributable to lighting changes. It was not until the early 1970s that the psychological community began taking a stronger interest in both pupillary movements and eye saccades. The technology of the time was limited but the researchers managed to acquire a significant knowledge base relating to the eye.
Aside from the obvious effect that light intensity has on pupil diameter, many emotional reactions can cause dilation or constriction. The primary emotional states associated with pupil dilation are excitement, comfort, pleasure, and displeasure. Pupil dilation also occurs in response to impending threat or pain. In general, more unappealing stimuli are responsible for pupil constriction. An example of this type of stimulus would be a cross-eyed baby. Some stimuli, such as concentration camp scenes, initially cause pupil dilation, but with repeated exposure cause pupil constriction.
In general, the pupil can range in diameter from 1.5 to over 9 millimeters, and can react to stimuli in as little as 0.2 seconds.
Reported methods of measuring pupil diameter include: using a short-focus telescope with adjustable cross hairs, or taking pictures of the eye as a stimuli is presented then later measuring the pupil diameter with a ruler. Although each of these methods will produce valid results, the extensive time investment required and potential for human error are significant problems.
With the increased interest in pupillometry, there has also been a corresponding increase in the interest directed towards saccadic eye movements. The different movements made by the eyes can be broken down into two main categories: voluntary fixation movements and involuntary fixation movements. The fixation movements, or jumps, are called saccades, while the movements are called opticokinetic movements. The saccadic jumps made by the eye are so quick that only 10 percent of the time is the eye engaged in movement, the rest is spent fixating.
The voluntary movements, called macro-saccades, are responsible for selecting the visual target of interest. Once a target has been acquired, the involuntary fixation mechanism assumes control. These involuntary movements, called micro-saccades, are responsible for maintaining target fixation.
Many different methods of tracking saccades have been reported in the literature. These methods range in technology from using the electrooculogram (EOG) to using magnetic fields to induce a current in a coil imbedded into a contact lens. Of the saccade tracking techniques reported in the literature, almost all required some sort of invasive interface with the subject. These invasions are often uncomfortable and inhibiting to the subject.